Restore math-italic ASCII fold for MathJax combining accents

_mathItalicCpToChar was simplified during rebase to a one-line
String.fromCodePoint(cp), on the theory that pt6's cmap aliases would
re-route U+1D400-block math-italic letters to the plain-letter glyphs
where the GPOS mark anchors live.  In practice the aliased path did
not trigger the mark feature for combining diacritics, so \hat{x},
\bar{H} etc. rendered with accents in the wrong place.

Restore the explicit U+1D434/U+1D44E/U+1D456 + U+210E + ASCII fold
from a3225f7 ("Fix the diacritics in the mathjax rendering") so the
injected text node carries plain ASCII letters that shape through the
font's mark anchors directly.

Also tighten the wide-mover overline position: anchor the hand-drawn
bar to baseRect.top with a small gap rather than overRect.top, which
includes the extra vertical padding MathJax allocates above the
stretched en-dash strip and pushed the bar visibly high above
\overline{a+b+c}.

Author: pelson

xkcd-script/samples/ligatures.png

@@ -1915,16 +1915,22 @@
         '2035': '̀',  // \grave     — reversed prime → combining grave
     };
 
-    // Math italic / bold / Greek codepoints (U+1D400 block, U+210E, etc.)
-    // are aliased to their plain Latin / Greek glyph in the font's cmap
-    // via pt6's _math_aliases.  So injecting the codepoint directly into
-    // a text node is enough — the browser shapes it through the font's
-    // cmap alias and gets the correct glyph.  Returns null for inputs
-    // that don't parse so the caller can skip cases the font won't
-    // cover (e.g. fancy decorative codepoints we never aliased).
+    // Map a hex glyph-class codepoint to a plain text character we can feed
+    // into a Unicode text node.  Math-italic letters (U+1D400 block) and
+    // the U+210E (italic h) outlier are folded back to ASCII A-Z / a-z so
+    // the browser shapes them via the font's plain-letter glyphs — the
+    // mark-anchor rules for combining diacritics attach to those, not to
+    // the math-italic codepoints, so going through pt6's cmap alias path
+    // produced mis-placed accents in practice.
     function _mathItalicCpToChar(hex) {
         const cp = parseInt(hex, 16);
-        return Number.isFinite(cp) ? String.fromCodePoint(cp) : null;
+        if (!Number.isFinite(cp)) return null;
+        if (cp >= 0x1D434 && cp <= 0x1D44D) return String.fromCharCode(0x41 + (cp - 0x1D434));   // A-Z italic
+        if (cp >= 0x1D44E && cp <= 0x1D454) return String.fromCharCode(0x61 + (cp - 0x1D44E));   // a-g italic
+        if (cp >= 0x1D456 && cp <= 0x1D467) return String.fromCharCode(0x61 + (cp - 0x1D44E));   // i-z italic (hole at U+1D455; h lives at U+210E)
+        if (cp === 0x210E) return 'h';
+        if (cp >= 0x20 && cp < 0x7F) return String.fromCharCode(cp);
+        return null;
     }
 
     function _glyphClassCp(el) {
@@ -2029,9 +2035,11 @@
             // letters it's covering, not the fallback over glyph.
             const left = baseRect.left - containerRect.left;
             const width = Math.max(baseRect.width, fontSizePx * 0.4);
-            // Vertical: just above the base's top; nudge so the bar/tip sits
-            // where the font's natural overline would.
-            const top = overRect.top - containerRect.top - barH * 0.5;
+            // Vertical: anchor to the base's top with a small gap so the bar
+            // sits where the font's natural overline / macron would.
+            // overRect.top is unreliable here — MathJax allocates extra
+            // vertical space above the en-dash ink in the over's bbox.
+            const top = baseRect.top - containerRect.top - barH - fontSizePx * 0.05;
 
             _hideOver(over);
             mover.dataset.xkcdWide = '1';